Horizontal Directional Drilling Project in New Caledonia: 128 m Power Duct Crossing Through Clay, Loess and Rock-Filled Ground
Successful Installation of 2 × 200 mm + 2 × 110 mm Power Ducts in Boulouparis
Drillto recently supported an underground power duct installation project in Boulouparis, New Caledonia, along Route Territoriale 1.
The project involved the trenchless installation of four power conduits-2 × 200 mm and 2 × 110 mm ducts-over a total crossing length of approximately 128 meters, with a straight drilling section of around 120 meters.
Although the crossing distance was relatively short, the project presented a number of technical challenges. The underground formation consisted of clay, loess and rock-filled backfill, with a particularly difficult section containing a large amount of backfilled rock at the deepest part of the crossing.
To maintain borehole stability and reduce the risk of pipe blockage during pullback, the construction team developed a targeted drilling fluid and reaming strategy, including the extensive use of bentonite and a controlled reduction of the reaming diameter before final pullback.
The entire pipe pullback operation was completed in approximately 1 hour and 30 minutes.
| Project Item | Details |
| Project Location | Route Territoriale 1, Boulouparis, New Caledonia |
| Application | Underground Power Duct Installation |
| Pipeline / Duct Configuration | 2 × 200 mm + 2 × 110 mm |
| Crossing Length | 128 m |
| Straight Drilling Length | 120 m |
| Maximum Crossing Depth | 9.2 m |
| Ground Conditions | Clay, loess and rock-filled backfill |
| Maximum Reaming Diameter | 600 mm |
| Pullback Pressure | 4–6 MPa |
| Rotary Pressure | 6–8 MPa |
| Pipe Pullback Time | Approx. 1.5 hours |
| Main Challenge | Deep section with substantial rock-filled backfill |
The biggest challenge of this project was not the overall 128-meter crossing distance.
It was the ground condition at the deepest section.
The planned drilling depth reached approximately 9.2 meters, and the deepest section contained a significant amount of backfilled rock.
According to the site conditions, some of the rock-filled material formed a zone approximately 1.2–1.5 meters in diameter.
This created several potential risks during HDD construction.
Borehole Instability
Loose or irregular backfilled rock can make the borehole wall less stable than more uniform soil formations.
If the surrounding material collapses into the borehole during pullback, it can increase friction and potentially restrict the movement of the product pipes.
Difficult Drilling Fluid Circulation
Rock-filled backfill can also affect the circulation and discharge of drilling fluid.
Maintaining sufficient drilling fluid circulation is important for carrying cuttings out of the borehole and supporting the borehole wall.
Increased Risk During Pullback
The most critical stage was the final pipe installation.
If the borehole became unstable after reaming, loose backfill material could collapse into the enlarged hole.
This could increase resistance around the pipes and create a risk of pipe sticking or pullback obstruction.
Therefore, the construction team needed to manage the borehole condition carefully before beginning the final pullback.
To address the difficult formation, the construction team adopted a combination of bentonite-based drilling fluid management, controlled reaming and reduced final pullback diameter.
The objective was not simply to create a large borehole.
It was to create a borehole that remained sufficiently stable throughout the transition from drilling to pipe installation.
Extensive Use of Bentonite for Borehole Protection
During the difficult crossing section, a relatively large amount of bentonite was used.
Bentonite is widely used in HDD operations as part of drilling fluid systems because it can help:
• Lubricate the borehole
• Support the borehole wall
• Carry drilled cuttings
• Improve drilling fluid circulation
• Reduce friction during pipe installation
• Improve hole cleaning
For this project, maintaining borehole stability was particularly important because of the loose rock-filled material at depth.
The drilling fluid was used to help maintain the integrity of the borehole while assisting with the removal of drilling cuttings.
The goal was to create a cleaner and more stable borehole before the final pullback operation.
Reaming Up to 600 mm
The project used a maximum reaming diameter of approximately 600 mm.
The reaming stage enlarged the pilot borehole to provide sufficient space for the power ducts and to prepare the hole for final installation.
However, simply maximizing the bore diameter was not considered the best approach for the final pullback because of the unstable backfilled rock section.
This is an important point in HDD construction:
The largest possible borehole is not always the safest or most efficient borehole.
Borehole diameter needs to be considered together with:
• Ground stability
• Product pipe size
• Drilling fluid performance
• Reaming conditions
• Pullback resistance
• Risk of borehole collapse
Final Pullback with a 400 mm Reamer
One of the key construction decisions was to change to a 400 mm reamer during the final pipe pulling operation.
The purpose was to avoid creating unnecessary space around the product ducts in the unstable rock-filled section.
A larger final hole could potentially leave more unsupported space within loose backfill, increasing the possibility of surrounding material collapsing into the borehole.
By using a 400 mm reamer for the pipe pullback stage, the team aimed to maintain a more controlled borehole condition and reduce the risk of:
• Borehole collapse
• Rock-filled material falling into the hole
• Excessive pullback resistance
• Pipe sticking
• Pipe damage
This adjustment demonstrates an important principle of HDD construction:
Equipment selection and drilling parameters should respond to actual geological conditions rather than follow a fixed procedure.
After the borehole preparation and reaming operations were completed, the final pullback operation was carried out.
The recorded operating parameters included:
• Pullback pressure: 4–6 MPa
• Rotary pressure: 6–8 MPa
• Pullback duration: approximately 1 hour 30 minutes
The four power ducts consisted of:
2 × 200 mm + 2 × 110 mm
The combination of appropriate drilling fluid management, borehole preparation, and final reaming strategy helped the team complete the pipe installation through the challenging ground conditions.
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From a distance, a 128-meter HDD crossing may appear to be a relatively short project.
However, HDD difficulty cannot be determined by distance alone.
A shorter crossing can become technically challenging when it involves:
• Significant depth
• Unstable ground
• Mixed geological formations
• Rock-filled backfill
• Existing infrastructure
• Limited working space
• Strict alignment requirements
This New Caledonia project is a good example.
The drilling distance was approximately 128 meters, but the deepest point reached 9.2 meters and included a difficult section of rock-filled backfill.
The project therefore required careful consideration of drilling fluid, borehole stability, reaming diameter and pullback conditions.
The project was carried out for underground power duct installation, an application where HDD technology can provide an effective alternative to conventional open-cut construction.
Installing power ducts using trenchless technology can be particularly useful when a pipeline route needs to cross existing infrastructure or when excavation would cause unnecessary disruption.
Typical HDD applications for power and utility infrastructure include:
• Underground power cables
• Electrical conduits
• Fiber optic cables
• Communication ducts
• Water pipelines
• Sewer pipelines
• Gas pipelines
For projects crossing roads and developed areas, horizontal directional drilling can reduce the need for continuous surface excavation while allowing utilities to be installed along a controlled underground alignment.
The New Caledonia project highlights several practical aspects of professional HDD construction.
Geological Conditions Matter
Machine selection and construction planning should always consider the actual ground conditions.
Clay, loess and rock-filled backfill can behave differently during drilling and pullback.
Drilling Fluid Is Part of the Construction Strategy
Bentonite is not simply an additive.
When correctly selected and managed, drilling fluid plays an important role in borehole lubrication, wall support and cuttings transport.
Reaming Diameter Should Be Controlled
A larger hole is not automatically better.
In unstable formations, excessive borehole enlargement can increase the risk of surrounding material collapsing into the bore.
Pullback Requires Its Own Planning
The conditions during final pipe installation can be different from those during pilot drilling.
The final reaming and pullback strategy should therefore be adjusted according to the actual borehole and geological conditions.
As an experienced manufacturer of Horizontal Directional Drilling Machines and trenchless equipment, Drillto focuses on developing equipment for a wide range of underground pipeline applications.
The company's product portfolio includes:
• Horizontal Directional Drilling Rigs
• HDD Drill Rods
• HDD Drilling Tools
• Pipe Pushing Machines
• Customized Trenchless Equipment
Drillto also provides technical guidance, operator training, machine maintenance support and construction assistance for international customers.
Different projects require different machine configurations, drilling tools and construction strategies.
For this reason, Drillto's approach is not simply to supply an HDD rig, but to understand the customer's pipeline requirements, crossing distance, geological conditions and construction environment before recommending an appropriate equipment configuration.
The successful completion of this project demonstrates how careful HDD planning can help contractors manage challenging underground conditions.
For this 128-meter power duct crossing, the combination of:
9.2 m maximum depth + clay and loess + rock-filled backfill + bentonite drilling fluid + controlled reaming + 400 mm final reamer
provided a practical approach to managing the most difficult section of the crossing.
The final pipe pullback was completed in approximately 1 hour and 30 minutes, bringing the project to a successful conclusion.
For Drillto, every overseas HDD project provides another opportunity to learn from real construction conditions and develop equipment and technical solutions that are better suited to the needs of contractors around the world.
Different ground. Different challenges. Different solutions.
